Subclass not accepted as input for method parameter?

I thought if I have a method that accepts classA (no super) as a parameter, and classA2 is a subclass of classA, then the method would also accept classA2 as a parameter.

Made a little test project, and yes, that works as expected.

Setting classA to a superclass (eg timer) - yep, still works.

However if I set the super to a different class in my project that is from a plug-in (and adjusting the parameters to accommodate the constructor) I get a TypeMismatch error “Expected pluginClass but got classA”

Any ideas on how this can happen?

concrete example ?
a subclass ISA superclass so I’m a bit lost why you’d be having this issue (whether the class is from a plugin or not)